Airborne satellite communication systems generally require an externally-mounted antenna unit. To achieve broadband data rates, a high-gain antenna is typically required, resulting in a significant aperture size. This structure is typically mounted on the top of the mobile platform, for example, on the crown of the fuselage of an aircraft. The structure is typically covered by an aerodynamically-shaped fairing having small frontal areas. Additional requirements are imposed by regulatory agencies to obtain spectrum authorization by the Federal Communications Commission (FCC) or equivalent in foreign jurisdictions.
One such requirement precludes interference with terrestrial wireless services. To provide a design that complies with FCC and European Telecommunication Standards Institute (ETSI) regulations, for example, requires reduction of the transmit antenna horizon and below-horizon sidelobe levels. FIG. 1 illustrates this problem. At a cruise altitude represented by line A, the sidelobe radiation of a crown-mounted antenna may irradiate the Earth, as represented by arc B. The sidelobe attenuation needs to be sufficient such that substantially no radiation extends beyond the arc labeled C. Current designs generally do not meet these requirements.
A solution proposed in the past has included an external choke plate to reduce radio frequency scattering near, at or below the horizon. This solution has not been acceptable for high-speed aircraft installations because such choke designs are exposed to the air stream and are susceptible to environmental issues such as corrosion and debris contamination. Such structures also have detrimental or unacceptable effects on aerodynamic drag, noise and vibration.
The apparatus and method of the present invention addresses considerations such as radiation pattern sidelobe requirements at, below and near the horizon for airborne mobile platforms to enable such mobile platforms to meet regulatory requirements.